The present invention relates to a process for preparing cis-1,3-diols. More particularly, the present invention relates to the use and subsequent recovery and reuse of a trialkylborane or dialkylalkoxyborane or a mixture of a trialkylborane and a dialkylalkoxyborane in the reduction of a beta-hydroxy ketone to obtain a cis-1,3-diol. Additionally, the present invention relates to the use of a synergistic combination of a trialkylborane and a dialkylalkoxyborane in the reduction of a beta-hydroxy ketone to obtain a cis-1,3-diol.
The use of trialkylboranes or dialkylalkoxyboranes in the stereoselective reduction of 1,3-keto alcohols to the corresponding syn-1,3-diols has been widely described in the literature. This method has given high stereoselectivity without using extraordinarily difficult conditions (Brower P. L., Butler D. E., Deering C. F., Le T. V., Millar A., Nanninga T. N., and Roth B., Tetrahedron Lett., 1992;33:2279; Narasaka K., and Pai F. C., Tetrahedron, 1984;40:2233; Chen K. M., Hardtmann G. E., Prasad K., Repic O., and Shapiro M. J., Tetrahedron Lett., 1987;28:155; Chen K. M., Gunderson K. G., Hardtmann G. E., Prasad K., Repic O., and Shapiro M. J., Chem. Lett., 1987:1923). There seems to be general acceptance of the formation of a borate ester from either the trialkyl or dialkylalkoxyboranes which is said to form a cyclic chelate (Narasaka K. and Pai F. C., Tetrahedron, 1984;40:2233; Chen K. M., Hardtmann G. E., Prasad K., Repic O., and Shapiro M. J., Tetrahedron Lett., 1987;28:155; Chen K. M., Gunderson K. G., Hardtmann G. E., Prasad K., Repic O., and Shapiro M. J., Chem. Lett., 1987:1923; see for example Paterson I., Cumming J. G., and Smith J. D., Tetrahedron Lett., 1994;35:3405). Axial delivery of a hydride to the complex then leads predominately to the syn-product which can be hydrolyzed to the diol. The diols are valued as intermediates for the preparation of, for example, HMG-CoA reductase inhibitors which are useful hypolipidemic and hypocholesterolemic agents. This is a widely used method of preparation of such agents (U.S. Pat. Nos. 4,645,854, 5,354,772, 5,155,251, and 4,970,313).
Many procedures in the literature, describe the work-up of the reaction with hydrogen peroxide (U.S. Pat. No. 4,645,854 and 4,970,313). This results in the destruction of active alkylborane species. Some procedures describe the repeated distillation with methanol and an acid (U.S. Pat. No. 5,354,772 and 5,155,251). This also dilutes and eventually destroys the active alkylborane species. We have surprisingly and unexpectedly found that by performing the reduction and workup with a minimal amount of acid, and keeping the distillate streams separated, that the initial distillate can be recovered and reused to obtain very good selectivity in subsequent reductions.
Thus, the present process offers significant advantages over the prior art processes. For example, the cost of additional alkylborane is eliminated for each batch in which the distillate stream is recycled. Additionally, since alkylboranes are hazardous, they must be destroyed prior to being disposed. The present process minimizes this expensive and time-consuming procedure. Moreover, it is especially surprising that very good selectivity in the reductions is obtained using recovered alkylboranes.
Finally, we have also surprisingly and unexpectedly found that a combination of a trialkylborane and a dialkylalkoxyborane is synergistic in selectively reducing a beta-hydroxy ketone to obtain a cis-1,3-diol.
Accordingly, a first aspect of the present invention is a process for the preparation of a compound of Formula I 
wherein R is alkyl,
NCxe2x80x94CH2xe2x80x94,
PGxe2x80x94Oxe2x80x94CH2xe2x80x94 wherein PG is a protecting group, 
which comprises:
Step (a) treating a compound of Formula II 
wherein R and R1 are as defined above with a trialkylborane or dialkylalkoxyborane or a mixture of a trialkylborane and a dialkylalkoxyborane in a solvent;
Step (b) adding an alkali metal hydride at about xe2x88x92110xc2x0 C. to about xe2x88x9250xc2x0 C.;
Step (c) concentrating the reaction by distillation to afford a compound of Formula I and a distillate containing alkylborane species; and
Step (d) treating additional compound of Formula II or III with the distillate from Step (c) containing recovered alkylborane species and repeating Steps (b) and (c) as desired to afford additional compound of Formula I.
A second aspect of the present invention is a process for the preparation of a compound of Formula I 
wherein R is alkyl,
NCxe2x80x94CH2xe2x80x94,
PGxe2x80x94Oxe2x80x94CH2xe2x80x94 wherein PG is a protecting group, 
which comprises:
Step (a) treating a compound of Formula II 
wherein R and R1 are as defined above with a synergistic combination of a trialkylborane and a dialkylalkoxyborane in a solvent; and
Step (b) adding an alkali metal hydride at about xe2x88x92110xc2x0 C. to about xe2x88x9250xc2x0 C. to afford a compound of Formula I.
A third aspect of the present invention is a synergistic combination comprising a trialkylborane and a dialkylalkoxyborane.
In this invention the term xe2x80x9calkylxe2x80x9d means a straight or branched hydrocarbon radical having from 1 to 10 carbon atoms and includes, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, secondary-butyl, isobutyl, tertiary butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, and the like.
xe2x80x9cPGxe2x80x9d means a protecting group used for protecting an alcohol moiety such as, for example, benzyl and the like. Additional examples of protecting groups for an alcohol moiety are disclosed at Chapter 2 in Greene T. W., xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d, John Wiley and Sons, Inc., 1981.
xe2x80x9cAlkali metalxe2x80x9d is a metal in Group IA of the periodic table and includes, for example, lithium, sodium, potassium, and the like.
xe2x80x9cAlkaline-earth metalxe2x80x9d is a metal in Group IIA of the periodic table and includes, for example, calcium, barium, strontium, and the like.
xe2x80x9cAlkali metal hydridexe2x80x9d includes, for example, sodium borohydride, zinc borohydride, lithium borohydride, lithium aluminum hydride, and the like.
xe2x80x9cAlkylborane speciesxe2x80x9d means a mono, di- or trialkylborane where the mono or dialkylborane is further substituted by hydrido or alkoxy as defined hereinafter or a dimeric alkylborane species.
xe2x80x9cAlkoxyxe2x80x9d means O-alkyl as defined above for alkyl.
As previously described, the compounds of Formula I are either useful as inhibitors of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG CoA reductase) or are useful as intermediates to prepare HMG CoA reductase inhibitors.
Thus, the present process can be used to prepare various HMG CoA reductase inhibitors containing a cis-1,3-diol moiety. For example, atorvastatin disclosed and described in U.S. Pat. Nos. 4,681,893 and 5,273,995; fluvastatin disclosed and described in U.S. Pat. No. 5,354,772; bervastatin disclosed and described in U.S. Pat. No. 5,082,859; cerivastatin disclosed and described in U.S. Pat. No. 5,177,080; NK-LO4 disclosed and described in U.S. Pat. No. 5,011,930; dalvastatin disclosed and described in U.S. Pat. No. 4,863,957; glenvastatin disclosed and described in U.S. Pat. No. 4,925,852; erythro-7-[5-(2,2-dimethylbutyryloxymethyl)-4-(4-fluorophenyl)-2,6-diisopropylpyridin-3-yl]-3,5-dihydroxy-6(E)-heptenoic methyl ester disclosed and described in U.S. Pat. Nos. 5,006,530, 5,169,857, and 5,401,746; 7,7xe2x80x2-[2-(dimethylamino)-4-(4-fluorophenyl)-6-isopropylpyridine-3,5-diyl]bis [erythro-(E)-3,5-dihydroxy-6-heptenoic acid methyl ester disclosed and discribed in U.S. Pat. No. 5,145,857; 7-[6-cyclopropyl-4-(4-fluorophenyl)-2-(4-methoxyphenyl)pyrimidin-5-yl]-3,5-dihydroxy-6(E)-heptenoic acid sodium salt disclosed and described in U.S. Pat. No. 5,026,708; (E)-7-[4-(4-fluorophenyl)-2-isopropylquinolin-3-yl]-3,5-dihydroxy-6-heptenoic acid xcex4-lactone disclosed and described in U.S. Pat. Nos. 5,011,930, 5,102,888, and 5,185,328; trans-(E)-6-[2-[2-(4-fluoro-3-methylphenyl)-6,6-dimethyl-4-(N-phenyl-carbamoyloxy)-1-cyclohexenyl]vinyl]-4-hydroxytetrahydropyran-2-one disclosed and described in U.S. Pat. No. 5,001,144; erythro-(E)-7-[2-(4-fluoro-3-methylphenyl)-4,4,6,6-tetramethyl-1-cyclohexen-1-yl]-3,5-dihydroxy-6-heptenoic acid sodium salt disclosed and described in U.S. Pat. No. 4,863,957; (E)-trans-6-[2-[2-(4-fluoro-3,5-dimethylphenyl)-4-hydroxy-6,6-dimethyl-1-cyclohexenyl]vinyl]-4-hydroxytetrahydropyran-2-one disclosed and described in U.S. Pat. No. 4,900,754; ethyl E-(3R,5S)-7-[4xe2x80x2-fluoro-3,3xe2x80x2,5-trimethyl(1,1xe2x80x2)biphenyl-2-yl]-3,5-dihydroxy-6-heptenoate disclosed and described in U.S. Pat. No. 4,567,289; 3(R),5(S)-dihydroxy-7-[4-(4-fluorophenyl)-1-isopropyl-3-phenyl-1H-pyrazol-5-yl]hept-6(E)-enoic acid disclosed and described in U.S. Pat. No. 4,613,610; and (3R,5S)-BMY-21950 disclosed and described in U.S. Pat. No. 4,897,490 can be obtained using the present process. All of the aforementioned U.S. patents are herein incorporated by reference.
The process of the present invention in its first aspect is an improved, economical, and commercially feasible method for preparing a compound of Formula I. The process of the present invention in its first aspect is outlined in Scheme 1. 
Thus, a compound of Formula II wherein R is alkyl,
NCxe2x80x94CH2xe2x80x94,
PGxe2x80x94Oxe2x80x94CH2xe2x80x94 wherein PG is a protecting group, 
or a compound of Formula III wherein R and R1 are as defined above is treated with about 0.1 to about 2.0 molecular equivalents of a trialkylborane such as, for example, triethylborane, tripropylborane, tri n-butylborane, tri sec-butylborane and the like or a dialkylalkoxyborane such as, for example, dimethylmethoxyborane, dimethylethoxyborane, dimethylisopropoxyborane, diethylmethoxyborane, diethylethoxyborane, diethylisopropoxyborane, diisopropylmethoxyborane, diisopropylethoxyborane, diisopropylisopropoxyborane, and the like or a mixture of a trialkylborane and a dialkylalkoxyborane as described previously, followed by the stereoselective reduction with about 1 molecular equivalent of an alkali metal hydride such as, for example, sodium borohydride, zinc borohydride, lithium borohydride, lithium aluminum hydride, and the like; in a solvent such as a hydrocarbon, for example hexane, toluene, cyclohexane and the like; an alkanol, for example methanol, ethanol, isopropanol and the like; or an ether, for example diethyl ether, tetrahydrofuran, 1,2-dimethoxyethane, triglyme (triethylene glycol dimethyl ether) and the like, or mixtures thereof at a temperature of about xe2x88x92110xc2x0 C. to about xe2x88x9250xc2x0 C. to afford after concentration by distillation a compound of Formula I. Additional compound of Formula II or III is subsequently treated with the distillate obtained by vacuum distillation from the first run followed by stereoselective reduction carried out as described above to afford a second batch of a compound of Formula I. Thus, recovered quantities of the alkylborane species can be used to convert additional quantities of a compound of Formula II or III to a compound of Formula I. This procedure using recovered alkylborane species can be repeated as desired to obtain additional quantities of a compound of Formula I.
Preferably, the reaction is carried out with about 1.2 to 0.8 molecular equivalents of triethylborane or diethylmethoxyborane or a mixture of a trialkylborane and a dialkylalkoxyborane as described previously in a solvent, preferably a mixture of tetrahydrofuran and methanol, at a ratio of about 8 volumes of tetrahydrofuran to one volume of methanol. This is followed by the addition of about one molecular equivalent of sodium borohydride at about xe2x88x92110xc2x0 C. to about xe2x88x9250xc2x0 C., preferably at xe2x88x9280xc2x0 C., followed by stirring for about 30 minutes to about 3 hours. Under these preferred conditions, greater than 90% of a compound of Formula I is produced in the desired stereochemical conformation.
Preferably, the present process is used to prepare [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate which is used as an intermediate to prepare atorvastatin. Compounds of Formula II or III are either known or capable of being prepared by methods known in the art.
The process of the present invention in its second aspect is an improved, economical, and commercially feasible method for preparing a compound of Formula I as previously outlined in Scheme 1.
In this aspect of the invention, applicants have found that a combination of a trialkylborane and a dialkylalkoxyborane surprisingly and unexpectedly is synergistic in selectively affording the desired cis-1,3-diol over the undesired trans-1,3-diol compared to the use of either a trialkylborane or a dialkylalkoxyborane alone. The synergistic combination comprises about 1% to 99% by weight of a trialkylborane and about 99% to 1% by weight of a dialkylalkoxyborane; preferably, a combination of about 90% by weight of a trialkylborane and 10% by weight of a dialkylalkoxyborane. This synergistic combination is of particular advantage since it does not require a stir time of the alkylborane species with the hydroxyketone at ambient temperature before reduction. The conditions and solvents for carrying out the reaction with a synergistic combination of a trialkylborane and a dialkylborane are as previously described above.
Thus, for example, in the preparation of [R-(R*,R*)]1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate when no pre-stir is used, triethylborane affords a 5 to 10:1 (cis:trans) mixture. When diethylmethoxyborane is used in place of triethylborane, a 5 to 10:1 (cis:trans) mixture is obtained. When a combination of 10% by weight of diethylmethoxyborane and 90% by weight of triethylborane is used, typically a greater than 30:1 (cis:trans) mixture is obtained. This synergistic effect of combining a dialkylalkoxyborane and a trialkylborane could not have been predicted based on the use of either reagent alone or literature precedent.
The following examples are illustrative to show the present processes and to show the usefulness in the preparation of (4R-Cis) 1,1-dimethylethyl-6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate which is an intermediate prepared from a 1,3-diol of the present process that can be converted to atorvastatin ([R-(R*,R*)]-2-(4-fluorophenyl)-xcex2,xcex4-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H-pyrrole-1-heptanoic acid, calcium salt (2:1)) (crystalline Form I) which is useful as a hypolipidemic and hypocholesterolemic agent.